260 by C.B. Dimalanta and G.P. Yumul, Jr. Crustal thickening in an active margin setting (Philippines): The whys and the hows National Institute of Geological Sciences, College of Science, University of the Philippines, Diliman, Quezon City, Philippines 1101. E-mail: [email protected] A synthesis of crustal thickness estimates was made parts of the Philippines could be attributed to subduction processes? recently utilizing available field, geochemical, seismic- What is the effect of collision events on crustal growth? What other processes contribute to the thickened crust? This paper forwards an ity, shear wave velocity and gravity data in the Philip- explanation on the different mechanisms and processes, which can pines. The results show that a significant portion of the account for the thickness of crust in the Philippines. This, hopefully, Philippine archipelago is generally characterized by can help in our understanding of how crustal growth processes occur in an active margin setting. crust with a thickness of around 25 to 30 kilometers. However, two zones, which are made up of a thicker crust (from 30 to 65 km) have also been delineated. The Tectonic setting of the Philippine Luzon Central Cordillera region is characterized by thick crust. Another belt of thickened crust is observed archipelago in the Bicol-Negros-Panay-Central Mindanao region. The convergence of the Sundaland-Eurasian margin with the Philip- This paper examines the interplay of tectonic and mag- pine Sea Plate resulted in the different features that comprise the matic processes and their role in modifying Philippine Philippine archipelago. These include magmatic arcs, subduction arc crust. The processes, which could account for the zones, collision zones and marginal basins. Based on seismicity and volcanism, the Philippine archipelago is divided into the seismically observed crustal thicknesses, are presented. The contri- active Philippine Mobile Belt and the aseismic Palawan microconti- butions of magmatic arcs as compared to the contribu- nental block. The Philippines is bounded on the east by the west- tion of the emplacement and accretion of ophiolite com- ward-dipping East Luzon Trough—Philippine Trench along which plexes to crustal thickness are also discussed. the Philippine Sea Plate is obliquely subducting. NUVEL-1 mea- surements show that the Philippine Sea Plate, in the region northeast of Luzon, is moving northwest at a rate of approximately 7 cm per year. But southeast of Mindanao, the plate motion increases to ~ 9 cm Introduction per year (Bautista et al., 2001). Several marginal basins are found on the western portion of the archipelago. These include the South The evolution of continental crust has been a problem of long stand- China Sea, Sulu Sea and Celebes Sea basins, which are subducting ing interest to various geoscientists. In studies, which try to address along the east-dipping Manila-Negros-Sulu-Cotabato trenches (Fig- the question of crustal growth, the addition of materials to the crust ure 1). The different ophiolite complexes, which can be found in var- is attributed to several mechanisms. Juvenile crustal material is pro- ious parts of the country, are believed to have originated from the duced in island arcs and oceanic plateaus (Rudnick, 1995; Condie, marginal basins or their ancient counterparts surrounding the archi- 1998). These features eventually collide with and become sutured to pelago. The magmatic arcs associated with the subduction zones as continents thereby leading to the growth of continental crust. Contri- well as the different ophiolite complexes are discussed in the suc- butions from magmatic underplating and overplating (Wilson, 1994; ceeding sections. Excess stress resulting from the subduction of Rapp and Watson, 1995) as well as from the emplacement of ophio- oceanic crusts along these subduction zones is absorbed by the lites have also been recognized (Dewey and Windley, 1981). Pro- Philippine Fault Zone and other related fault structures (Fitch, 1972). cesses such as arc magmatism, oceanic plateau accretion, intraplate volcanism, crustal underplating, accretion of ophiolites, sediment Magmatic arcs subduction, tectonic erosion and delamination may operate singly or in combination to contribute to crustal growth. In the Philippines, The Philippine island arc is built on oceanic crust, which had relatively few studies have looked closely into the problem of crustal been modified by several episodes of magmatism (Figure 2a). Arc growth. Crustal thickness estimates are limited to specific areas and magmatic activity that characterized the evolution of the Philippines are mostly derived from geochemical, seismicity, focal mechanism served to thicken the crust of this island arc system. and gravity data (Bautista et al., 2001; Barretto et al., 2000). A recent The earliest volcanic activity recorded in arc volcanic rocks compilation of geochemical, seismicity and gravity data to estimate from the Philippine Mobile Belt commenced during the Cretaceous crustal thickness in various parts of the Philippines show that a sig- time (Wolfe, 1981; Deschamps and Lallemand, 2002). A record of nificant portion of the Philippine archipelago is characterized by this volcanic activity is preserved in arc rocks from Cebu, which crust with thicknesses ranging from 15 to 30 km. However, areas yielded a late Early Cretaceous age (~108±1 Ma) (Walther et al., underlain by a thicker crust (from 30 to 65 km) have also been rec- 1981). Evidence for this activity is also preserved in Catanduanes ognized from a recent compilation (Dimalanta & Yumul, 2003). Island where an andesite sample gave a K-Ar age of 121.09±2.61 Ma The complex geodynamic and tectonic setting of the Philip- (David, 1994). There is also a record of magmatism preserved in the pines necessitates an understanding of the role of tectonic and mag- Luzon Central Cordilleran region during the Late Cretaceous period matic processes to crustal growth. The following questions need to (K-Ar dating of schist sample: 82.6±20.6 m.y.) (Wolfe, 1981). Rin- be addressed. How much of the observed crustal thickening in some genbach (1992) has also reported an Upper Cretaceous age (Cam- December 2004 261 panian-Maastrichtian) for the pelagic foraminifera found in volcani- clastic sediments in Angat, Southern Sierra Madre (Figure 1). The subduction of the Philippine Sea Plate along the proto-East Luzon Trough is believed to be responsible for this magmatic episode. This is consistent with the existence of a well-developed accretionary prism, which cannot be attributed to the rejuvenated, present-day East Luzon Trough (Balce et al., 1976; Yumul et al., 2003c). A Late Cretaceous volcanic arc sequence was mapped in the Caramoan region in southeastern Luzon. 40Ar-39Ar dating of amphi- bole separates from a basaltic lava flow sample yielded an age of 91.1±0.5 Ma (David et al., 1997). Cretaceous magmatism is also recorded in Rapu-rapu where diorite intruding the ultramafic rocks yielded a 77.1±4.6 Ma age (David et al., 1996). Further south of that, three magmatic episodes have been recognized in the Southeastern Luzon volcanic arc. The oldest magmatism is recorded at 68.6 Ma from Balatan, which is in the central portion of the Southeastern Luzon volcanic arc. This age was based on the result of a K-Ar dat- ing on a dioritic sample (Japan International Cooperation Agency—Metal Mining Agency of Japan, 1999). Andal (2002) sug- gested that this magmatic episode might be attributed to the subduc- tion of the northern margin of the Indo-Australian Plate below the Philippine Sea Plate. In Central Philippines, the oldest dated rocks are found in Guimaras Island with an age of 59±2 Ma (Wolfe, 1981) (Figure 1). The next most significant period of magmatism is represented by the Paleogene (Paleocene-Oligocene) and Neogene magmatic belts (Figure 2a) (Wolfe, 1981; Bureau of Mines and Geosciences, Figure 1 Map of the Philippines showing its general tectonic 1982; Yumul et al., 2003b). Most of the Paleogene magmatic belts features. The Philippines is divided into the seismically active are found in eastern Philippines (e.g. Sierra Madre, Bicol, Samar- Philippine Mobile Belt (horizontally lined pattern) and the Leyte and eastern Mindanao). Volcanism along eastern Philippines, aseismic Palawan microcontinental block (dark gray shaded from Bicol to Leyte, is mostly related to the westward subduction of area). The rate and direction of convergence as determined from the Philippine Sea Plate along the Philippine Trench (Divis, 1980). the GPS measurements (adopted from Bautista et al., 2001) is Oligo-Miocene magmatism, which are significant because of the indicated by the vectors. Areas from which Cretaceous magmatic gold deposits in the Philippines that are related to these intrusions, ages were determined (using K-Ar or Ar-Ar dating = filled circle; are represented by rocks in the Luzon Central Cordillera, eastern using paleontologic dating = filled square) are also shown on the Negros—western Panay, Eastern Mindanao and Cotabato (Figure map (Walther et al., 1981; Wolfe, 1981; David, 1994; David et al., 2a) (Mitchell and Leach, 1991; Yumul et al., 2003a). 1996; 1997). Figure 2 A—Map showing the ancient and modern magmatic arcs (modified from Aurelio, 2000; Yumul et al., 2003b). B—Map showing the different ophiolite/ophiolitic complexes that comprise the Philippine archipelago (Yumul, 2003). C—Map summarizing the thickness of the crust in various parts of the Philippines. Area enclosed in dashed lines corresponds to crustal thickness of ~15 to 29 kilometers. Gray shaded areas indicate crustal thickness from 30 to 65 kilometers (modified from Dimalanta and Yumul, 2003). See text for details. Episodes Vol. 27, no. 4 262 The Neogene magmatic arc from Tablas–Western Panay, which was interpreted by Mitchell and Leach (1991) to be a contin- Crustal growth by magmatic and uation of the Miocene Luzon arc, is deemed to be associated with the subduction of the South China Sea plate along the Manila-Negros tectonic processes Trench.